Name:
How Green are Hydrogen and Corn-Based Fuels?
The students are allowed to work together.
Some politicians and news sources tout hydrogen and corn-based fuels as being clean,
environment-friendly, and/or sustainable alternatives to fossil fuels. You will evaluate these
claims using the following worksheet. In the future, when you hear of new green technology,
you will know how to evaluate how green the technology is using the skills learned from this
worksheet. VERY IMPORTANT: some of the problems below involve numerous questions. Do
not become overwhelmed by this. Instead, answer the first question and then move onto the
next one.
Hydrogen Fuel
When hydrogen fuel is burned the following reaction occurs:
H2(g) + O2(g)  H2O(g)
1. Using the standard reaction enthalpy tables in your textbook, how much energy would
be released from burning one mole of hydrogen gas?
ΔHo = -241.82 kJ/mol
Hydrogen gas is touted as a green technology based on the above reaction. As you can see,
the only product is water. However, hydrogen does not exist naturally in large quantities nor
does it exist in a way that it can be easily harvested. Instead, hydrogen has to be produced.
Below is the most used industrial method for producing hydrogen (~95% of hydrogen gas is
produced this way):
Step 1: CH4(g) + H2O(g)  3 H2(g) + CO(g)
Step 2: CO(g) + H2O(g)  CO2(g) + H2(g)
2. Now that you know the above information, recall that on the warm-up you did one of
the statements was “Could hydrogen, the simplest and most abundant element in the
universe, address both energy and clean air concerns?. For the statement in bold, what
do you think the purpose of this statement is? I other words, the item in bold has
nothing to do with hydrogen as a fuel yet the author of the article must have included it
for a reason. What is that reason?
This is an example of misinformation. It is arguably true that hydrogen is the simplest
molecule. It is also true that it is the most abundant element in the universe. However, its
simplicity will not make it any greener since, even though it is the most universal element in the
universe, it is not the most universal element on our planet (in fact, hydrogen is very reactive
with oxygen and, furthermore, any hydrogen that doesn’t react with oxygen will eventually
float off into space so there really is no natural source of hydrogen on this planet that hydrogen
can be harvested from in appreciable quantities.)
3. Using the standard reaction enthalpy tables, calculate the enthalpy change for Step 1
and Step 2 above.
For Step 1 ΔHo = +206.1 kJ/mol
For Step 2 ΔHo = -41.16 kJ/mol
4. Based on your answer from problem 3, what does this mean in terms of energy? Do the
reactions require energy or give off energy? If a reaction requires energy, where do you
think this energy will come from?
Since ΔHo for Step 1 is positive, it requires energy to make hydrogen gas. Step 2 actually gives
off energy but not enough to compensate for Step 1. Thus, energy needs to be supplied to the
reaction of Step 1 to get it to go. This energy will most likely come from a fossil fuel so, in
addition to producing CO2 from the actual production of hydrogen gas, CO2 will be produced
from the fuel needed to make the hydrogen gas.
To burn octane, the primary component of gasoline, the following reaction occurs:
2 C8H18(l) + 25 O2(g)  16 CO2(g) + 18 H2O(l)
5. Determine how much energy is released by the burning of one mole of gasoline using the
standard reaction enthalpies found in your book.
ΔHo = -5074.56 kJ/mol
6. Based on your answers to problems 1 and 4, how many moles of hydrogen gas are
equivalent to one mole of gasoline? Based on your answer to problem 2, how much
energy is required to produce the amount of hydrogen that is equivalent to one mole of
gasoline? How much gasoline would this require? How much CO2 has been produced so
far? How does the amount of CO2 made so far in the production of hydrogen gas
compare to the burning of one mole of CO2?
[Approximately 21 moles of hydrogen gas are equal to one mole of gasoline (-5074.56/-241.82).
21 moles of hydrogen gas means Step 1 and 2 above would need to occur about 5.25 times (21
moles of hydrogen needed/4 moles of hydrogen produced every time a cycle of step 1 and 2 is
performed). Thus, ~1082 kJ of energy is needed for Step 1 while Step 2 releases ~259.1 of
energy. All together, ~823 kJ of energy is needed to produce the energy of one mole of
gasoline. This would require ~0.16 moles of gasoline (823/5074.56).
All together, ~6.55 moles of CO2 have been produced (every cycle of Step 1 and Step 2 produces
one mole CO2. So, since there were 5.25 cycles of CO2, 5.25 moles of CO2 were produced
directly from producing hydrogen. From the gasoline needed to heat the reaction: 0.16 moles
of gasoline times 8 moles of CO2 produced for every mole of gasoline burned gives ~1.4 moles
of CO2). Considering one mole of gasoline produces 8 moles of CO2, the production of hydrogen
gas looks hardly green.]
7. An important piece of information was not reported above. That piece of information is
that Step 1 does not occur at room temperature. Instead, the reaction has to be heated
to ~950 K (we will see how to calculate this temperature towards the end of this unit
after we discuss “entropy” and “Gibbs free energy”. When we get to that point,
calculate the minimum temperature the reaction has to be heated to. Don’t worry
about using this real temperature value in any of the other calculations for this
worksheet). Where do you think the energy to heat the reaction comes from?
[It will take ~67 kJ of energy to heat one mole of water (if one cycle of Step 1 is performed) to
950K. (The specific heat capacity of liquid water is 0.07524 kJ/K mol. Since one mole is being
heated from 298 K to 373 K, this requires 5.64 kJ of energy. The enthalpy of vaporization of
water is 40.7 kJ/mol so 40.7 kJ of energy is needed to vaporize one mole of water. To heat
water vapor from 373 K to 950 K, 20.9 kJ of energy is needed since the specific heat capacity of
water vapor is 0.03618 kJ/K mol. Adding this all together gives ~67 kJ of energy)
This energy would likely come from a fossil fuel like oil. Since 5.25 cycles of Step 1 need to be
performed, 351.75 total kJ of energy are needed (5.25 times 67 kJ). This would require 0.07
moles of gasoline and would produce 0.555 more moles of CO2 for a current total of ~7.1 moles
of CO2. Considering each mole of gasoline produces 8 moles of CO2, the “greenness” of
hydrogen gas is dropping.
After Gibbs free energy and entropy have been discussed the students should be able to
calculate the total change in Gibbs free energy (142.12 kJ/mol) and the total change in entropy
(0.21462 kJ/K mol). Since the Gibbs free energy value is positive, the reaction does not occur at
room temperature; the reaction will only occur once the reaction reaches a negative value.
Thus, the minimum temperature that this occurs at is right after Gibbs free energy equals zero.
Using the Gibbs free energy equation along with the enthalpy value of 206.1 kJ/mol, the
minimum temperature needed for step 1 to occur is 960K. Thus, the calculations done using
950K slightly underestimates how bad the production of hydrogen gas is.]
8. What phase of hydrogen is made in steps 1 and 2? What is the problem with this phase
when it comes to storing hydrogen in a car? What phase do you think is better for
storing hydrogen and why? What would it take to store the hydrogen in this phase (i.e.
the temperature of the phase change)? Will this require energy? If it does, where will
this energy come from?
The gas phase of hydrogen is made in steps 1 and 2. There are several problems when hydrogen
is in this phase. First, as a gas, the storage space needed would be much larger than a typical
gas tank. Secondly, hydrogen is highly flammable and explosive in the gas phase. Thus, I would
convert the hydrogen to a liquid phase. Considering that the freezing point of hydrogen is 20 K
(-252 oC), a lot of energy would be needed to liquefy hydrogen gas. Also, a car using hydrogen
gas would have to use some of the hydrogen gas to keep the rest of the hydrogen in a liquid
state. Overall, the energy used to liquefy the hydrogen gas will come from fossil fuels.
9. Based on the above answers, do you feel water vapor is the only product from using
hydrogen gas as a fuel, as is often claimed by some politicians and news outlets? What
other products are produced while producing the hydrogen gas?
Water vapor is clearly not the only product produced by hydrogen cars. Yes, the hydrogen cars
themselves may only output water but the process that produces hydrogen also expels CO2.
Considering that the above calculations showed that hydrogen is barely greener than gasoline,
and considering that the above calculations involve ideal conditions (which don’t exist), it is
highly likely that hydrogen gas is less green than gasoline (currently, this is so) due to the extra
energy wasted because of non-ideal losses. (However, when you discuss this assignment after it
has been handed in, you should note that another way to produce hydrogen gas involves using
electricity to split water into hydrogen gas and oxygen gas. Thus, if we could make large
improvements in converting solar energy into electricity, hydrogen gas could become a very
green source of energy.)
10. On a separate piece of paper, write at least one paragraph discussing how the article
you read presented how green hydrogen fuel is. Also discuss how what you learned
from doing this worksheet altered your opinion about what you wrote for the Hydrogen
Fuel Warm-up (make sure to review their responses to the warm-up ASAP so that you
can hand it back to the students for this part of the assignment). Then write at least one
paragraph discussing the information I gave to you in terms of, in the future, how would
you find out that information yourself? Lastly, discuss any questions you feel are left
unanswered.
Answers will vary.
Corn-Based Fuel
The big claim is that the US should switch from gasoline to ethanol produced from corn.
The corn doesn’t produce ethanol directly but it does produce sugars. These sugars are
harvested and are then used by anaerobic bacteria (don’t require oxygen) to turn the sugar into
ethanol. Many states are getting large subsidies from the government to do this. Part of the
claim is that ethanol would be more efficient and produce less CO2, thus reducing global
warming. You will determine if this is a good idea or not. Ethanol is C2H6O and, as you learned
before, gasoline can be treated as pure octane, C8H18.
Burning of Ethanol
C2H6O(l) + 3 O2(g)  2 CO2(g) + 3 H2O(l)
Burning of Octane
2 C8H18(l) + 25 O2(g)  16 CO2(g) + 18 H2O(l)
11. Using the standard reaction enthalpies in your book, calculate how much energy is produced
from burning one mole of ethanol and from burning two moles of octane.
Burning of one mole ethanol ΔHo = -1,365.15 kJ/mol
Burning of two moles of octane ΔHo = -10,941.3 kJ (-5,470.65 kJ/mol)
11. How much energy, in kJ, is released by the combustion of 1.00 g of ethanol?
MW of ethanol is 46 g/mol. 1 g divided by MW is 0.0217 moles. Multiply this by -1,365.15
kJ/mol and you get 29.6 kJ released.
12. How much energy, in kJ, is released by the combustion 1.00 g of octane?
MW of octane is 114 g/mol. 1 g divided by MW is 0.00877 moles. Multiply this by -5,470.65
kJ/mol and you get 48.0 kJ released.
13. One gallon of gasoline has 23.2 mol of octane in it. To travel the same distance
traveled with one gallon of octane, how many moles of ethanol would you need? (Assuming
the car works just as well with ethanol as it does with gasoline and that the efficiency of
converting chemical energy to mechanical work is the same for both fuels.)
23.2 mols of octane produce -126,919.08 kJ of heat (23.2 times -5470.65). It would take ~93.0
moles of alcohol to produce the same amount of energy as one gallon of octane. (-126,919.08
divided by -1,362.15)
14. How many moles of CO2 are produced by one gallon of octane?
Since one mole of octane produces 8 moles of CO2, 23.2 moles would produce 186 mols
of CO2.
15. How many moles of CO2 are produced by the amount of ethanol equivalent to one
gallon of gasoline?
Since 93.0 moles of alcohol are equivalent to 23.2 moles of octane, energy wise, 186 mols of
CO2 are produced from ethanol. Thus, both fuels produce the same amount of CO2.
16. Which one is lighter to transport in your car, the gallon of octane or the weight of
the ethanol that is energetically equivalent to the gallon of octane?
Since octane produces the same amount of energy but at a lesser weight (23.2 moles of octane
weighs 2,645 g vs. 93 moles of ethanol at 4,278 g), it would be easier to transport octane.
17. So, based on these calculations are you convinced ethanol is a good deal? Is it
greener than gasoline? Is corn better used for feeding people or for being converted to
gasoline?
I don’t believe the above calculations are sufficient enough to decide the above
questions. It definitely takes less trips to the gas station to use octane. However, determining if
corn-based ethanol is greener than gasoline can’t be determined from the above calculations.
This is because, even though the energy used to produce corn is taken from the sun, I know
that some energy is required to turn the corn into ethanol and that this energy is supplied by
fossil fuels. The above calculations are not sufficient enough to know how much energy is
needed to turn corn into ethanol. (actually, corn based ethanol is barely better than gasoline.
However, there is hope that newer methods of producing ethanol will reduce CO2 emissions of
corn-based ethanol by 90% compared to gasoline). So, why give an assignment that can’t fully
be answered? The purpose of this is that, in addition to questioning the authority of
newspapers and politicians, I feel students should question the authority of teachers too. If
students are led to believe there is an answer because of the way the question is worded, then
this gives them a chance to question what they are being asked to do.
18. Lastly, do you feel more confident in your ability to analyze claims of sustainability
regarding fuels? Why? Yes?